The present invention concerns a cardiac pacer having a novel system for detecting the evoked response from a stimulation pulse applied to a chamber of the heart.
Closed loop cardiac pacing can be employed for the adjustment of stimulus intensity and rate by assessing the heart's electrical response to stimulation. This can be achieved either by employing additional electrodes for detecting the evoked response from the heart, or by electrically neutralizing the stimulating electrodes immediately after a stimulus to enable them to sense the evoked response. This neutralization is necessary because the delivery of the stimulus leaves the electrodes in a polarized state. In the presence of this depolarization it is difficult or even impossible to detect the evoked response without the electrode neutralization process.
The neutralization process involves charge compensation; it is a rapid restoration of the electrodes to their resting baseline potential. If the stimulus is capacitively coupled to its load, discharging the coupling capacitor in the reverse direction should restore the electrodes to their baselines. However, this process is too slow to allow detection of the evoked response which occurs well within the first 50 ms.
When some resistive elements are shunted by the closure of a switch following a stimulus, then the neutralization is accelerated by the shortened time-constant. This method is often referred to as "charge dump." This is neither sufficiently rapid, nor sufficiently complete for reliable detection of the evoked response.
It is, therefore, an object of the present invention to provide a system for detecting the evoked response in which the neutralization process is rapid, using a biphasic waveform technique, i.e., using a compensating current pulse that is transmitted in the opposite direction from the stimulating current pulse.
However, we have discovered that when a compensating current pulse fullows a stimulating pulse, the cathode current threshold rises. This implies increased current drain from the battery even beyond the charge necessary for compensation. It is, therefore, a further object of the present invention to reduce battery drain notwithstanding the use of a biphasic waveform technique.
Other objects and advantages of the present invention will become apparent as the description proceeds.